Is Sleep Quality More Important than Sleep Duration for Public Health?

In this issue of SLEEP, Clark and colleagues contribute an extremely large and well-conducted longitudinal study on the relationship between poor sleep and cardiovascular risk.¹ Using three waves of data from the Finnish Public Sector Study and linkage to electronic health records, the study authors examined if the onset of poor sleep was associated with the development of hypertension, diabetes, and dyslipidemia in over 45,600 working-aged adults. The onset of poor sleep was the transition to short or long sleep duration from 7–8 hours of “normal” sleep, or the change from negligible to more than weekly sleep disturbance over the course of 4 years. The outcomes were based on medication reimbursements after physician confirmation of the diagnosis with the follow-up period being up to 7 years after the onset of poor sleep.

Clark and colleagues' comprehensive analyses showed that the transition to short or long sleep duration was not significantly linked to the development of hypertension, diabetes, or dyslipidemia. The onset of sleep disturbance, however, predicted a roughly 20% risk increase for both hypertension and dyslipidaemia, with no additional risk conferred for diabetes. These findings for hypertension are generally consistent with results from previous meta-analytic reviews,^2,3 although the null findings for diabetes are surprising and suggest the diabetes risk associated with poor sleep may be more modest than previously estimated.^4,5 Importantly, the study contributes new data on the relationship between poor sleep and subsequent dyslipidemia, an association for which there is much less evidence. Previous longitudinal studies point to worse lipid profiles with short sleep duration^6–8 and worse sleep quality,⁹ and together with the rigorous results presented by Clark and colleagues, support a mechanistic chain connecting poor sleep to subsequent cardiovascular disease via dyslipidemia.

There is much to like about the study by Clark and colleagues, including its ability to investigate both sleep duration and sleep quality as health risk factors, its long follow-up, its large sample size, and ability to control for many factors that contribute to cardiovascular risk including smoking, physical activity, body mass index, and existing physical and psychiatric conditions. By including only participants with normal sleep durations and no sleep disturbance at the beginning of the study, and excluding those with a history of sleep apnea, heart disease, stroke, as well as the outcomes, the authors ensure that the development of poor sleep occurs before the development of disease, thus bolstering the case for causality in the findings.

The authors also conducted an impressive and thorough set of sensitivity analyses, including but not limited to considering relative rather than absolute changes in sleep duration to account for individual- and age-related differences in sleep need, excluding participants with preexisting use of sleep medications, and restricting the study population to a healthy subset without any history of chronic disease. Results from all of these point to the robustness of the main findings: (1) that sleep disturbance prospectively and independently predicts the development of hypertension and dyslipidaemia and (2) that the quality of sleep appears a more important risk factor than the quantity of sleep.

That sleep quality may be just as important, if not more important, than sleep duration in predicting future health is often overlooked. Meta-analyses show the relative risk increase associated with short sleep and hypertension and diabetes to be 20% and 30%, respectively, while the same associations for poor sleep quality range from 5% to 20% for hypertension and upwards of 40% for diabetes, depending on how sleep disturbance is operationalized.^5,10 Furthermore, there is very strong evidence tying sleep quality to the development of mental disorders: sleep disturbances more than double the risk of depression¹¹ and anxiety.^12–15

The conceptualisation of sleep as the distinct factors of duration and quality appears to have begun in 1964 when Hammond observed a relationship between sleep duration and mortality in the first American Cancer Society Cohort.¹⁶ This study revealed that those who had reported 7 hours sleep had the lowest mortality after 2 years, with deaths increasing on either side of this nadir. The study was one of the earliest, if not the earliest, to imply that there existed an optimal sleep duration, and that shorter and longer sleep durations were “unhealthy.” These results appear to have driven subsequent research on sleep duration and health outcomes. The study also examined the predictive value of poor sleep quality and found that insomnia was associated with excess mortality in men but not women.¹⁶ However, sleep quality was largely neglected until 1989, when Ford and Kamerow published a landmark study demonstrating that insomnia greatly magnified the risk of subsequent psychiatric disorders.¹⁴

As a result of this conceptual distinction, we have much evidence connecting sleep duration to physical health outcomes, and linking sleep quality to mental health, but know relatively little about the impact of sleep disturbances on the development of physical disease, or the contribution of short and long sleep durations to mental health. It may be the case that sleep disturbances are more important for psychological outcomes, with sleep duration possibly playing a stronger role for physical disorders, but this remains to be demonstrated by further research.

The distinction between quality and duration might also explain why population data on both sleep quality and sleep duration are rarely available from the same source. As a consequence, exploration of how sleep quality and sleep duration combine or interact to affect health has been limited. The conceptual split of sleep into quality and duration appears so well-established that even when information on both aspects of sleep are available, it is unusual to see them considered together. To be clear this is not a criticism of the excellent study by Clark and colleagues but a reflection of how duration and quality are conceptualised so distinctly that we may have forgotten they are measures of the same underlying phenomenon.

An understanding of the role of both sleep quality and sleep duration becomes doubly important when we are reminded that they are inextricably linked. People with short and long sleep durations are also those most likely to report sleep disturbances,^17–19 with the same U-shape commonly seen for health outcomes found in the relationship between sleep duration and poor sleep quality. It is likely that the extremes of sleep duration capture poor sleep quality rather than objectively short or long durations and that poor sleep quality is at least partly responsible for many of the U-shaped associations seen between sleep duration and health risks.

A handful of studies show that the effects of sleep duration and sleep quality on health outcomes are not simply additive,^20,21 and support the case for considering both facets of sleep where possible. Importantly, public health guidelines and interventions for sleep duration will necessarily be based on population norms and averages, and will not take into account the large individual differences in sleep need. In contrast, interventions to improve sleep quality may help all individuals, albeit some to a greater degree than others.

Sleep quality and sleep duration remain valuable measures of sleep because of their cost-effectiveness, low participant burden, and intuitive interpretation. Re-integrating the concepts of sleep quality and sleep duration to examine their independent and combined impact on health outcomes appears a sensible and necessary step for understanding the contribution of sleep as a whole to public health.

CITATION

Bin YS. Is sleep quality more important than sleep duration for public health? SLEEP 2016;39(9):1629–1630.

DISCLOSURE STATEMENT

Dr. Bin has indicated no financial conflicts of interest.